(use-package polymode
  :ensure t
  :config
  (use-package poly-R)
  (use-package poly-markdown)
  ;;; MARKDOWN
  (add-to-list 'auto-mode-alist '("\.md" . poly-markdown-mode))
  ;;; R modes
  (add-to-list 'auto-mode-alist '("\.Snw" . poly-noweb+r-mode))
  (add-to-list 'auto-mode-alist '("\.Rnw" . poly-noweb+r-mode))
  (add-to-list 'auto-mode-alist '("\.Rmd" . poly-markdown+r-mode))
  (markdown-toggle-math t)
  ;; from https://gist.github.com/benmarwick/ee0f400b14af87a57e4a
  ;; compile rmarkdown to HTML or PDF with M-n s
  ;; use YAML in Rmd doc to specify the usual options
  ;; which can be seen at http://rmarkdown.rstudio.com/
  ;; thanks http://roughtheory.com/posts/ess-rmarkdown.html
  (defun ess-rmarkdown ()
    "Compile R markdown (.Rmd). Should work for any output type."
    (interactive)
                                        ; Check if attached R-session
    (condition-case nil
        (ess-get-process)
      (error
       (ess-switch-process)))
    (let* ((rmd-buf (current-buffer)))
      (save-excursion
        (let* ((sprocess (ess-get-process ess-current-process-name))
               (sbuffer (process-buffer sprocess))
               (buf-coding (symbol-name buffer-file-coding-system))
               (R-cmd
                (format "library(rmarkdown); rmarkdown::render("%s")"
                        buffer-file-name)))
          (message "Running rmarkdown on %s" buffer-file-name)
          (ess-execute R-cmd 'buffer nil nil)
          (switch-to-buffer rmd-buf)
          (ess-show-buffer (buffer-name sbuffer) nil)))))
  )

(with-eval-after-load 'polymode
  (define-key polymode-mode-map (kbd "<f5>") 'ess-rmarkdown))

Symbol's value as variable is void: polymode-mode-map